[hpsdr] Questions re internal and external LPF on Penelope, Penny Lane and Hermes

[Kjell Karlsen]

Hi John.

I have been following the development of Hermes and have done the  
measurements of the output spectrum. I found that the output from the PA  
had too much noise and spurious due to pick up of direct and harmonics of  
the clock oscillators by the amplifier.

The bandwidth of the PA is very high. To limit the bandwidth, 2 capacitors  
were inserted on U7 inputs.

In addition, LFCV-52+ was included after the PA. After this modifications,  
the alias from 50- 52 MHz, is >-50 dBc, at 53 MHz it is -45 dBc and at 54  
MHz it is -40 dBc. The spurs and noise are very low, >60 dB down except  
the 2.nd harmonic that is -40 dBc at 51 MHz. This means that Hermes can be  
connected to the antenna  without any extra outboard filtering at 6 meter.

On the lower bands, the harmonics are on not suppressed by the LPF on the  
output so be careful. On 20 meter, the 2.nd and 3.rd harmonics are only  
-30 dBc.

If a PA is connected, a LP filter is needed. Harmonics are always  
generated by the PA and must be removed to meet the FCC requirements.

It is not possible to use Hermes on higher order alias frequencies without  
taking the signal directly from the DAC. There are no jumpers or other  
provisions on the PCB to do so. FL1 must be removed to get access to T1.

The same is with the RX. There is no way of direct connection to the  
Pre-amp without removing FL6.

I hope this answers most of your questions concerning Hermes. I have not  
measured on Penny or Penny Lane.

By the way, we are working on a new and hopefully improved PA for a future  
"Hermes PRO"

73, Kjell



På Tue, 06 Nov 2012 05:53:00 +0100, skrev John Marvin  
<jm-hpsdr at themarvins.org>:

> ***** High Performance Software Defined Radio Discussion List *****
>
>   I was looking at a FAQ on Penelope that addressed why Penelope has no  
> filtering on the output. That got me interested in looking at how things  
> have changed going from Penelope to Penny Lane to Hermes:
>
> 1) Without any filtering I would think that there would have been a fair  
> amount of aliased output. In some cases people use that as a feature,  
> i.e. deliberately use the aliasing to generate desired signals in ranges  
> above the Nyquist rate. That was even mentioned as a possible feature of  
> Penelope.  But at the same time it was claimed that at 0.5 W output,  
> Penny Lane meets the FCC requirements without additional filtering. How  
> far down was the signal in the Fs/2 - Fs range compared to the DC - Fs/2  
> range?
>
> 2) So, after all the good arguments for no filtering, two filters were  
> added to Penny Lane, an RLP-40+ and what appears to be series LC  
> band-stop discrete filter. Were the original arguments not valid, or was  
> there another reason for doing this?
>
> 3) Then yet another LPF filter was added to Hermes (an LFCV-52+). Why  
> was this added?
>
> 4) The Penelope FAQ claimed that no external filtering was needed at  
> 0.5watt, yet the user guide for Hermes claims that an external LPF is  
> always needed, even with all the additional filtering.  Is that actually  
> true? If so, what are the sources of out of band signals that exceed the  
> FCC requirements that didn't exist on Penelope?
>
> 5) Has anyone done any detailed measurements on Hermes TX output?
>
> 6) Has anyone ever tried doing VHF/UHF receive and/or transmit work on  
> Mercury and or Penelope/Penny Lane using undersampling/aliasing  
> techniques? If so, how and what was done (e.g. filters bypassed, FPGA  
> work done, etc.).  Has there ever been any consideration of adding  
> jumpers or other features to make it easier to bypass the input and  
> output LPF filters in order to explore this possibility (which would  
> require external band pass filtering)?
>
> Thanks!
>
> John
> AC0ZG
>
>
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